Scanning tunneling microscope (STM)-induced selective bond breaking in individual molecules and conventional STM imaging are combined to determine the nature of chemisorbed O-2 species formed during the initial stages of silicon (111)-(7x7) oxidation. A selective atomic-scale modification mechanism that involves dissociative electron attachment of tip-emitted electrons to empty adsorbate orbitals is introduced. Two molecular species were found : one involves O-2 bonded to an already oxidized silicon adatom, and the other involves an O-2 molecule that is bonded to a second-layer rest atom and interacting with two silicon adatoms.